Fe Isotope Variations in the Modern and Ancient Earth and Other Planetary Bodies

摘要

Iron, the fourth most abundant element in the Earth's crust, has four naturally occurring stable isotopes: ~(54)Fe (5.84%), ~(56)Fe (91.76%), ~(57)Fe (2.12%), and ~(58)Fe (0.28%), and the natural, mass-dependent isotope variations of Fe in the rock record span a range of ~4 per mil (‰) in ~(56)Fe/~(54)Fe ratios (Fig. 1). The field of Fe isotope geochemistry is relatively new but has received considerable attention because it may allow us to gain a better understanding of how Fe is cycled in different environments. Iron typically occurs as either reduced ferrous Fe in oxygen-poor environments, or as oxidized ferric iron in oxygen-rich environments. Notably, only the reduced species is soluble in oxygenated aqueous solutions, unless the pH is low. In the Archean and Early Proterozoic, the earth may have been relatively oxygen-poor (e.g., Kasting et al. 1979; Grandstaff 1980; Holland 1994), suggesting that there may have been significant quantities of Fe (0.9 millimolar) dissolved in the oceans as Fe(Ⅱ)_aq (e.g., Ewers 1983; Sumner 1997). The extensive iron formations of Archean to Early Proterozoic age may have been deposited from such Fe(II)-rich oceans (e.g., Beukes and Klein 1992). In the modern oxic oceans, however, Fe contents are exceedingly low, <1 nanomolar in the open oceans (e.g., Martin and Gordon 1988; Bruland et al. 1991; Martin 1992; Johnson et al. 1997), and it is now recognized that marine productivity is Fe-limited in parts of the open oceans (e.g., Martin and Fitzwater 1988; Martin et al. 1989, 1994). The differences in the behavior of Fe with redox state, and the significant isotope fractionations (l‰ or more in ~(56)Fe/~(54)Fe) that are associated with redox conditions, suggests that Fe isotope studies will be extremely useful for tracing the Fe geochemical cycle.